In fiber optic systems and certain other applications, an optical subassembly couples the light from a semiconductor laser into an end face of an optical fiber. Reflections from anywhere within the optical sub-assembly, such as the fiber end face, optical lens element, beam splitter, polarizer or optical isolator, may provide feedback to the laser. Unfortunately, semiconductor lasers, including vertical cavity surface emitting lasers (VCSELs), can be very sensitive to optical feedback. Medium to strong feedback in the range of about −35 dBm to 0 dBm may give rise to relative intensity noise (RIN), power modulation, or other coupled cavity effects.
Conventionally these effects are addressed through careful design of the optical package in which the devices are housed. Current approaches include the use of angled fiber end facets, fiber anti-reflective (AR) coatings, lens AR coatings, defocusing along the optical axis, beam splitters, and optical isolators. However, the cost of adding or modifying external optical elements is typically higher than the cost of integrated components. Therefore, these approaches, if used only to address the problem of optical feedback, may increase the cost of the optical package.
Conventional device designs further exasperate the problem of optical feedback. For example, the transmission in a conventional mirror is roughly the same in each direction. The reflectivity, however, is typically asymmetric and often is higher looking from the air toward the cavity, than from cavity to air. Conventional mirrors may therefore strongly return reflections from one or more external components and may therefore create a high Q external cavity when the laser is integrated into an optical sub-assembly. When fluctuations in laser drive current or temperature occur, the external cavity acts as a Fabry-Perot etalon which modulates the output power. In addition, two high Q cavities in series can cause multiple longitudinal modes to appear, which can give rise to RIN.
Referring to FIG. 1, Applicant of the present invention previously integrated an absorptive layer 8 into the emitting mirror of a VCSEL 10 to reduce the reflectivity of the emitting mirror as seen by a feedback optical wave. The VCSEL 10 included a lower mirror 14 formed above a substrate 12, an optical cavity 16 formed above the lower mirror and an upper mirror formed 18 above the optical cavity. The upper mirror 18 of this device was a hybrid mirror, having a semiconductor portion 20 and a dielectric portion 22. The device further included a current confinement ion implant 24 as well as a current constriction 26 for mode control and defining individual devices on a wafer.
The dielectric portion 22 of the hybrid mirror comprised alternating one-quarter wavelength thick layers of a high index of refraction dielectric material and a low index of refraction dielectric material. In this approach an absorptive titanium layer 28 was formed at the low-to-high index transition closest to the emitting facet. In this embodiment the titanium layer 28 was processed to remove it from within the aperture formed by an upper ohmic contact 30 to reduce the absorption losses as seen from the cavity. However, this approach provides less absorption of the optical feedback as seen from the external cavity. In particular, the large number of longitudinal modes that appear in the transmission spectrum due to the external cavity is not reduced.